A massive battery installed at one of the authority’s substations will store electricity generated by the braking systems on trains (as the trains slow down the wheels drive generators). The battery will help trains accelerate, cutting power consumption, and will also provide extra power that can be sold back to the regional power grid. The pilot project, which involves one of 38 substations in the transit system, is expected to bring in $500,000 a year. This figure would multiply if the batteries are installed at other substations.

The project shows how cash-strapped public transit agencies that operate major subways and electric rail systems could find a new source of income by tapping into the smart grid. It also highlights one way that the smart grid could save energy, avoid blackouts, and incorporate more renewable power.

Several energy utilities are already experimenting with using large batteries to help smooth out fluctuations in electricity supply, keeping the grid operating at the correct frequency and preventing blackouts. In the new pilot project, this kind of battery technology will be paired with software from a Philadelphia-based smart grid company called Viridity Energy.

The software will decide how to allocate the power stored in the battery, using it to drive trains and smooth out spikes in SEPTA’s power distribution network. SEPTA could also sell the excess power to various electricity markets, depending on a number of factors, including what’s most profitable.

The pilot project, which Viridity expects to complete by next summer, will make use of the regenerative braking capabilities of Philadelphia’s subway trains. These systems convert kinetic energy into electricity as the train slows down. In the existing system, without the battery, that power can be sent back into SEPTA’s power distribution system and used to help another train accelerate–but only if it happens to be accelerating at the same time the power is generated, says Laurie Actman, director of government relations and business development at Viridity. Otherwise that power is lost.